Curable resin compositions comprising epoxy resins are utilized as resins having excellent heat resistance in the fields of construction, civil engineering, automobiles, aircrafts and the like. Even in the field of semiconductor-associated materials, the epoxy resins used in electronic devices are required to have superior characteristics, and also, in recent years, utilization of epoxy resins in the field related to optoelectronics engineering is attracting public attention.
Display apparatuses such as liquid crystal displays, plasma displays, EL displays and portable devices have been distributed to general consumers, and in addition to the demand for size increase, weight reduction and thickness reduction, there is also an increasing demand for display in curved surfaces, three-dimensional display and the like. In the display elements of such apparatuses or in optical members such as a front panel, glass plates have been widely used in order to meet various requirements such as transparency, hardness, chemical resistance, and gas barrier properties. However, glass plates have a problem that they are susceptible to cracking and are heavy. Thus, in order to solve this problem, plastic materials such as epoxy resins have been examined as a substitute of glass plates, and various suggestions have been made.
For example, Patent Literature 1 describes a transparent resin substrate for liquid crystal display elements, which uses an epoxy resin, an acid anhydride-based curing agent, and an alcohol. Furthermore, Patent Literatures 2 and 3 describe transparent substrates each using a glass cloth and a heat-curable resin, and Patent Literature 4 describes a resin sheet using a cured resin layer comprising a cloth-like body made of glass fiber and inorganic particles.
Glass substitute plastic materials including these are prone to undergo warpage or cracking as a result of shrinkage at the time of curing in the production process thereof, and it is difficult to obtain smooth sheets. Furthermore, since glass substitute plastic materials have larger coefficients of linear expansion as compared with glass plates, problems may occur due to expansion or shrinkage even at the time of use, and further, sufficient performances that are required in the market as substitutes of glass, such as color, heat resistance, light resistance and hardness, are not obtained. Even in regard to the methods suggested in the documents described above, it cannot be said that the methods give satisfactory results in terms of color. Particularly in connection with the slight increase in refractive index, many of the substrate materials have low values of the transmittance at a short wavelength of near 400 nm, and thus it can be seen that coloration into yellow color or the like is exhibited. When a substrate material is used for optical applications, it is required that the material have a high transmittance at near 400 nm. For example, in the methods suggested in Patent Literatures 2 and 3, the transmittance of visible light at 400 nm is less than 90%. In the method suggested in Patent Literature 4, the transmittance is a transmittance of visible light at 550 nm, but the value is still less than 90%.
The problem of warpage or cracking at the time of production and the problem of expansion or shrinkage at the time of use may be ameliorated by using a glass cloth or an inorganic filler in combination. However, in order to obtain transparency, it is necessary to have the refractive index of the glass cloth or inorganic filler fit to the refractive index of the resin, there are limitations on the materials that may be used, and thus it becomes difficult to satisfy other properties.
Examples of the curing agent for epoxy resins that are generally used in such fields include acid anhydride-based compounds. Particularly, acid anhydrides formed from saturated hydrocarbons are frequently utilized because cured products thereof have excellent light resistance. Common examples of these acid anhydrides include alicyclic acid anhydrides such as methyltetrahydrophthalic anhydride, hexahydrophthalic anhydride, and tetrahydrophthalic anhydride, and among them, methylhexahydrophthalic anhydride, methyltetrahydrophthalic anhydride and the like, which are liquid at normal temperature, are primarily used from the viewpoint of easy handleability.
However, since the alicyclic acid anhydrides described above have high vapor pressures, when an epoxy resin is cured in an open system by using one of these compounds as a curing agent for epoxy resin, as this compound itself volatilizes in the atmosphere, there occur not only environmental contamination and adverse effects on human body caused by the release of hazardous compounds to the atmosphere, but there is also a problem that contamination of the production line, and curing failure of the epoxy resin composition caused by the absence of a predetermined amount of a carboxylic acid anhydride (curing agent) in the cured product, may occur. Furthermore, the characteristics of an epoxy resin composition are changed to a large extent by the curing conditions, and it is difficult to obtain a cured product having the intended performance in a stable manner.
Particularly, in optical applications, it is necessary to have the refractive index of an inorganic material such as a glass cloth fit to the refractive index of the resin in order to obtain transparent sheets. However, if the curing agent in the resin volatilizes in a large amount at the time of curing, an intended refractive index may not be obtained, and there occurs a serious problem that defective transparency is obtained.
Anyways, it is difficult to have transparency that may substitute glass, reduction of shrinkage at the time of curing, and the conditions of color, heat resistance, light resistance, hardness and smoothness altogether, and a material that may satisfy all of these conditions has not been obtained yet.